Electro-optic materials, i.e. materials whose refractive index (RI) varies with application of an electric field currently are known, an example of such a material being Lithium Niobate (LiNbO.sub.3). The existence of the electro-optic property provides for the development of various optical/light transmitting devices whose RI may be controlled by the application of an electric field.
Lithium Niobate waveguide modulators are commercially available but there are limitations inherent in relation to the existing technology. Fabrication of devices employing Lithium Niobate is complex and the materials are expensive. Its high dielectric constant is not entirely appropriate for the realisation of high speed, low voltage devices and the differences between Lithium Niobate and silica optical fibre both in terms of linear refractive index and of waveguide geometry make low loss coupling to standard fibre systems difficult. Furthermore, at short (e.g. blue) wavelengths lithium niobate suffers from photorefractive damage.
By far the most practical and readily available material for optical devices is silica or silicate glass (referred to herein for convenience as "silica glass" or "glass"), but unfortunately, silica glass displays little or no electro-optic properties.
An attempt has been made to enhance electro-optic properties in doped silica glass devices by heating the devices in the presence of an applied electric field. This has had some effect but the resultant electro-optic effect has proved to be insufficient for practical purposes.
Attempts have also been made at enhancing the electro-optic properties of doped silica glass utilising visible light in the presence of an applied electric field. "Generation of Permanent Optically Induced Second-Order Nonlinearities in Optical Fibers by Poling" by Bergot et al appearing in Optics Letters, Volume 13, No. 7, July 1988 at pages 592-594 (Bergot et al) discusses a process of inducing a second order optical nonlinearity in germanosilicate fibres by applying a transverse DC electric poling field in the presence of a high intensity light. Bergot et al discloses utilising a pulsed laser operating at 485 nm and a CW argon laser operating at 488 nm. It further discloses launching light from these lasers axially into the core of the fibre in the presence of a DC electric field. This has had some effect but again, the resultant electro-optic effect has proved insufficient for practical purposes.